Conventionally, electromagnetic linear motors having cams, links or the like for converting a rotation into a linear movement or solenoid motors were used for obtaining a linear driving force. However, there is a problem with a linear motor, which uses cams or the like for converting rotation into a linear movement since the size is extremely large and the movement accuracy is very low. Further, there is a problem with a solenoid motor in that linear displacement is limited since the solenoid must be disposed around a total linear displacement. Thus, in order to solve these problems, a piezoelectric motor is moved to the front. The piezoelectric motor has a simple structure compared to the electromagnetic linear motor. Further, it can be driven at a low speed. Moreover, its displacement is less limited than the solenoid motor.
Generally, when several sheets of piezoelectric elements (which thin metal sheets are interposed between) are overlapped with each other, a sufficient amount of electricity can be obtained. Mechanical deformation can be converted into an electric signal due to such character of a piezoelectric element. The character has been used in a microphone, record player, etc. for a long time. On the other hand, if a high frequency voltage is applied to a piezoelectric crystallized plate, the plate will expand and contract periodically. In particular, if the frequency of the voltage is matched to a natural frequency of the plate, then it will resonate to thereby oscillate strongly. This is referred to as a reverse piezoelectric effect. Strong and stable mechanical oscillation can be obtained by this reverse piezoelectric effect. The piezoelectric linear motor uses oscillation according to the reverse piezoelectric effect for obtaining a linear driving force.
Further, piezoelectric/electrostriction motors are generally driving sources without any magnets and winding wires. Since a piezoelectric motor is driven at a low speed with a strong driving force without any complex parts for force transfer as a gear, cam and the like. Also, it is not influenced by a magnetic field with simple structure. Since the sound wave produced during an oscillation of the motor is an inaudible ultra-sound wave, it can be driven silently. Further, its displacement can be controlled in an accurate nanometer scale.
The piezoelectric linear motor can be classified into two types. One is a motor driven by a progressive wave produced by the flexural wave. Another is a motor having an actuator driven by a combination of a longitudinal stationary wave and a lateral stationary wave, while the actuator makes a linear displacement with repetitive longitudinal and lateral oscillations. In the former motor driven by the progressive wave, a stator formed from a piezoelectric ceramic with a metallic elastic body attached thereto contacts a mover, while the elastic body attached to the stator produces an oscillating wave for driving the linear motor. The latter motor driven by the stationary waves is driven by the oscillating wave of the piezoelectric body during an application of a voltage. The voltage has a frequency corresponding to the natural frequency of each stator. This motor has a higher efficiency and a much simpler control circuit.
As shown in FIGS. 1 and 2, the piezoelectric element 1 may be deformed while a piezoelectric element 1 is connected to an electric power supply 30 for applying a voltage to a piezoelectric element 1. FIGS. 1 and 2 show the deformations of piezoelectric elements having different polarizing directions. The polarizing direction of a piezoelectric element shown in FIG. 1 is opposite to that of FIG. 2. Here, P represents a polarizing direction and E represents an electric field direction. If the electric field E is applied, then the piezoelectric element 1 will expand or contract. If the polarizing direction of the piezoelectric element 1 matches the electric field, then the piezoelectric element 1 will expand in the z direction and contract in the x direction as 1′. On the other hand, if the polarizing direction of piezoelectric element 1 is opposite to the electric field, then the piezoelectric element 1 will expand in the x direction and contract in the z direction as 1″.
As shown in FIGS. 3 and 4, if the piezoelectric element 1 is coupled to a metallic elastic body 10, then the expansion or contraction of the piezoelectric element 1 will bend them, although they have different elasticities. In FIG. 3, the polarizing direction of the piezoelectric element 1 is coincident to the direction of the electric field. Thus, the piezoelectric element 1 and the elastic body 10 are bent to the −z direction. On the other hand, as shown in FIG. 4, the polarizing direction of the piezoelectric element 1 is opposite to the direction of the electric field. Thus, the piezoelectric element 1 and the elastic body 10 are bent to the z direction. FIGS. 1 to 4 illustrate a lateral oscillation. However, a longitudinal oscillation can be illustrated in the same way.
As shown in FIGS. 5 and 6, a conventional piezoelectric linear motor comprises a piezoelectric element 1, an elastic body 10 as a stator coupled to the piezoelectric element 1, a power supply 30 applying AC voltage to the piezoelectric element 1 and a mover 20. The mover 20 is moved by an oscillation of the elastic body 10.
In the conventional piezoelectric linear motor, a voltage application makes stationary oscillations of the piezoelectric element 1. Its lateral oscillation makes the elastic body 10 repeat hard contact to the mover and becoming loose therefrom. Its longitudinal oscillation makes the elastic body oscillate in an orthogonal direction to the linear movement of the mover 20. That is, as shown in FIG. 5, the elastic body 10 contacts the mover 20 by a lateral oscillation of the piezoelectric element 1, while the elastic body 10 directs as an arrow shown in FIG. 5 by a longitudinal oscillation. Thus, the mover 20 is moved linearly in the direction of the arrow by the movement of the elastic body 10. On the other hand, the elastic body 10 is moved away from the mover 20 by the lateral oscillation, while the elastic body 10 is moved in an opposite direction to the mover movement by the longitudinal oscillation. At this time, the mover 20 may not move. As such, the mover substantially moves in the direction of the arrow shown in FIG. 5 since the combination of the longitudinal oscillation and the lateral oscillation is produced repeatedly.
However, there is a problem with the prior art piezoelectric linear motor in that the contact between the elastic body and the mover causes an extreme abrasion, thereby reducing the durability.
There is a further problem with the prior art piezoelectric linear motor in that the movement of the mover cannot be controlled accurately since a transmission of the displacement from the elastic body to the mover may be changed by the abrasion.
There is another problem with the prior art piezoelectric linear motor in that right and reverse movements cannot be achieved since there is only one oscillation mode with the combined oscillation of the lateral and longitudinal oscillations in one piezoelectric element.